Many alloying uses of metallic nickel require high purity. This is particularly true when metallic nickel is used in forming superalloys. Small amounts of impurities, e.g. tens of parts per or even parts per million, can produce highly detrimental properties. For example, sulfur in amounts greater than 50 parts per million and selenium in evan smaller amounts, i.e., more than about 2 parts per million, can induce hot shortness in superalloys, causing problems during hot working. Metallic nickel can be produced by electrorefining, electrowinning, carbonyl techniques and by precipitation of metallic nickel with a reducing gas from aqueous solutions. The first three of these processes can generally produce a refined nickel product that contains less than 5 ppm selenium. Metallic nickel precipitated from aqueous solutions generally contains about 5 ppm and 40 ppm selenium, and most often between about 10 ppm and about 20 ppm selenium. But even 1 ppm selenium in nickel can be critically detrimental when the nickel is used in the production of special superalloys.
Some selenium is removed by partial volatilization when nickel is produced with intermediate pyrometallurgical treatments. Selenium can also be partially removed by coprecipitation with other hydroxide when iron is precipitated from solutions under oxidizing conditions in a pH range between about 4 and 6. Some selenium can also be removed by cementation with metallic copper at moderate temperatures and with metallic nickel at temperatures above about 200.degree. C. Some selenium can also be removed from nickel solutions with ferric hydroxide produced by hydrolysis at temperatures above 200.degree. C.
These known processes for removing selenium from nickel solutions are very expensive, only partially effective or inapplicable. For example, those processes that require the use of temperatures of 200.degree. C. are frequently commercially unattractive because such reactions require the use of pressurized vessels in a large production stream, can entail the loss of nickel from solution or are effective in removing only selenium that is present in the tetravalent state. These known processes which rely on the coprecipitation of selenium with hydrolyzed ferric hydroxide or other hydroxides are not applicable to the treatment of pure nickel solutions because either these solutions do not contain the coprecipitating compound or are effective in only removing tetravalent selenium. A process for removing both tetravalent and hexavalent selenium from nickel solutions under moderate operating conditions has now been discovered.